The present disclosure relates to a current-feedback amplifier and more specifically to a current-feedback amplifier having reduced crossover distortion with phase delay eliminated.
FIG. 1 depicts a generalized current-feedback amplifier (CFA). The circuit of FIG. 1 includes an input stage 10 and an output stage 20. The input stage 10 includes an input port (IN) providing a signal to the base of an NPN bipolar junction transistor (BJT) 104 and a PNP BJT 116. The collector of the transistor 104 is connected directly to a first common voltage line V+, while its emitter is connected through a current sink 114 to a second common voltage line Vxe2x88x92. The collector of the transistor 116 is connected directly to Vxe2x88x92, while its emitter is connected through current sink 120 to V+.
The input stage 10 further includes emitter follower transistors 124 and 132. A transistor 124 has a base connected to the emitter of transistor 116, while transistor 132 has a base connected to the emitter of transistor 104. The collector of the transistor 124 is connected to an input of a first current mirror 128. The collector of the transistor 132 is connected to an input of a second current mirror 136. The emitter transistor 124 and the emitter of transistor 132 are connected to form a common node n10. The current mirror 128 has an output connected to the output of the current mirror 136 at node n20. A capacitor 140 connects node n20 to ground.
In the output stage 20, the base of transistor 142 is connected to node n20, while its emitter is connected to V+ through a current source 146 and its collector is connected directly to Vxe2x88x92. The base of a transistor 150 is also connected to node n20. The collector of the transistor 150 is connected directly to V+, while its emitter is connected to Vxe2x88x92 through a current sink 154.
The output stage 20 further includes emitter follower transistors 158 and 160. The base of transistor 158 is connected to the emitter of transistor 142, and the base of transistor 160 is connected to the emitter of transistor 150. The collector of transistor 158 is connected to V+, while the collector of transistor 160 is connected to Vxe2x88x92. The emitter of transistor 158 and transistor 160 are connected together to form an output node n30. The output node n30 is connected by a feedback resistor 172, having a value RF, to node n10. A load resistor 176, having a value RL, connects the output node n30 to ground.
FIG. 2 depicts a simplified CFA that results if the fifth and sixth BJTs are replaced by diodes. FIG. 2 depicts an input stage 10A that is identical to the input stage 10 of FIG. 1. Note that components carried over from FIG. 1 to FIG. 2 are similarly labeled, as will be components carried over from FIG. 1 or other figures into subsequent figures. In the output stage 20A, the capacitor 140, transistors 142 and 150 and current sources 146 and 154 have been removed between the two current mirrors 128 and 136, relative to FIG. 1. The diodes 240 and 242 are connected in series between the current mirrors 128 and 136. The base of transistor 158 now connects to the output of the current mirror 128, while the base of transistor 160 connects to the output of current mirror 136.
The diodes 240 and 242 of the simplified CFA of FIG. 2 are replacements for emitter-followers used in the design of FIG. 1. Eliminating emitter-followers eliminates phase delay over frequency due to limited FT in the transistors. Therefore, the potential bandwidth may be extended beyond that of the classic CFA which includes emitter-followers. However, the downside of the diode replacement is that there is less current gain around the feedback loop. Nevertheless, reduced current gain increases the amplifier""s output impedance and provides less suppression of internal distortion.
Often such a simplified CFA, as shown in FIG. 2, is used as an output stage within an overall amplifier. The bandwidth increase in the simple CFA allows more bandwidth in the externally compensated overall amplifier. Furthermore, the simple CFA is also much more linear than a simple emitter-follower output stage.
FIGS. 3 and 4 depict half sine wave input signals 300 and 400. The positive sine wave signal 300 is formed from a normal sine wave signal with all negative values of the sine wave attenuated. The negative sine wave signal 400 is formed from the same normal sign wave as is used to form the positive wave signal 300, however in the negative sine wave signal, all positive values for the sine wave are attenuated. The positive sine wave signal 300 is passed to the positive current mirror 128 and the negative sine wave signal 400 is passed to the negative current mirror 136. Each current mirror 128 and 136 replicates the signal on the output of the particular current mirror into which the signal is input.
Dynamic problems occur when trying to pass these half-sine waves 300 and 400. While devices will traverse trajectories of on-state to off-state fairly faithfully, it is very difficult for a device to immediately traverse from fully-off to suddenly on. In particular, when a current mirror is turned off, voltages across the devices composing the current mirror lag to small values at rates limited by the capacitances within the devices of the current mirror. Thus, there will be enough time for the current mirror""s devices to turn fully off in the off half-cycle of current. From the fully off state, the current mirror is called upon to turn on during its off half-cycle. To pass the current in an undistorted manner, the voltages across the devices within the current mirror must come to an on-state virtually immediately. However, capacitances of the device terminals prevent that immediate change of voltage. Thus, the output of the current mirror will not accurately respond to the signal. This signal distortion caused by the rapid change in states demanded of the current mirrors is a form of cross-over distortion. At every zero crossing of output current the switching of current from one set of devices, an output error will be introduced into the signal due to the lag in current mirrors"" response in switching from a fully off state to a fully on state. Therefore, what is needed is an amplifier that produces an amplified signal exhibiting less distortion at zero cross-over points.
The disclosure describes a current feedback amplifier (FIG. 5) output stage that contains an additional pair of emitter follower transistors 538 and 540 with a capacitor 548 connecting the emitters of transistors 538 and 540 to ground to reduce discontinuities in the output current. The introduction of the pair of transistors 538 and 540 and the capacitor 548 allows each current mirror 128 and 136 to be turned on prior to the time the particular current mirror is required to control the output of the amplifier. By having the non-dominant current mirror capacitively turned on prior to the time it is required to dominate the output signal allows the non-dominant current mirror to more accurately replicate the input signal at the time it is required to dominate the output. Thus, signal glitches due to switching on of the current mirror, as occur in classic AB amplifiers as shown in FIG. 2, are avoided and less signal distortion results in the output signal.